Titanium dioxide coatings on magnesium alloys for biomaterials:
A review
1
Vanessa Hernández-Montes a, Claudia Patricia Betancur-Henao a & Juan Felipe Santa-Marín b
a
Facultad de Ciencias Exactas, Instituto Tecnológico Metropolitano, Medellín, Colombia. [email protected], [email protected]
b
Facultad de Ingeniería, Instituto Tecnológico Metropolitano, Medellín, Colombia. [email protected]
Received: August 18th, 2016. Received in revised form: December 5th, 2016. Accepted: February 13th, 2017
Abstract
Magnesium and its alloys are used for biomaterials in orthopedic applications. Such alloys are still under development, and they are used due to their
biocompatibility and mechanical (bone-like) properties that make them suitable to be used as biomaterials. Magnesium has potential to be used in
biodegradable implants because of its capacity to support tissue regeneration processes. Therefore, multiple strategies have been developed to enhance
magnesium properties. Coatings on magnesium alloys have been used to improve the cytocompatibility and corrosion resistance of magnesium.
Particularly, titanium dioxide can be used as coating on magnesium to help regulating degradation rate and overcome some issues when magnesium is
inserted into the human body. Accordingly, this paper is a critical review to consolidate the available literature about titanium dioxide coatings on
magnesium alloys for potential use as biomaterials. The work focuses on coatings obtained by the sol-gel route as a promising technique for biomedical
applications.
Keywords: Magnesium alloys; titanium dioxide; surface modification; coatings, biomedical.
Recubrimientos de dioxido de titanio sobre aleaciones de magnesio
para biomateriales: Una revisión
Resumen
El magnesio y sus aleaciones, que aún se encuentran en desarrollo, se usan en aplicaciones ortopédicas debido a su biocompatibilidad y propiedades
mecánicas (similares al hueso), que los hacen adecuados para aplicaciones en biomateriales. El magnesio tiene potencial para ser usado en implantes
biodegradables dada su capacidad de soportar los procesos de regeneración de tejidos. En consecuencia, se han desarrollado varias estrategias para
mejorar las propiedades del magnesio. Los recubrimientos sobre magnesio se emplean para mejorar su citocompatibilidad y resistencia a la corrosión.
Específicamente, el dióxido de titanio se puede usar como recubrimiento protector sobre el magnesio, con el fin de ayudar a regular la velocidad de
degradación y superar algunos problemas que se encuentran cuando el magnesio es implantado en el cuerpo. Por tanto, en este artículo se realizó una
revisión crítica para consolidar la literatura disponible acerca de recubrimientos de dióxido de titanio sobre aleaciones de magnesio para potenciales
aplicaciones en biomateriales. En este documento se hace énfasis en los recubrimientos obtenidos por medio de la de la ruta sol-gel como técnica
prometedora para aplicaciones biomédicas.
Palabras clave: Aleaciones de magnesio; dióxido de titanio; modificación superficial; recubrimientos; biomédico.
1. Introduction
Biomaterials are either natural or synthetic materials used
to replace or assist an organ or tissue. They are in direct
contact with biological systems [1] and they must be
biocompatible. Biocompatibility was defined in 2008 as "the
ability of a material to perform the required function in
response to medical therapy without causing undesired
effects at either the local or systemic level in a user. The
material must also produce a proper cell or tissue response in
order to create relevant clinical therapy" [2]. Moreover, a
biocompatible material can be inert or can interact and react
with the tissue. Other biocompatible materials are degraded
How to cite: Hernández-Montes, V., Betancur-Henao, C.P. and Santa-Marín, J.F., Titanium dioxide coatings on magnesium alloys for biomaterials: A review, DYNA 84(200),
pp. 261-270, 2017.
© The author; licensee Universidad Nacional de Colombia.
DYNA 84 (200), pp. 261-270, Marzo, 2017. Medellín. ISSN 0012-7353 Printed, ISSN 2346-2183 Online
DOI: http://dx.doi.org/10.15446/dyna.v84n200.59664
Hernández-Montes et al / DYNA 84 (200), pp. 261-270, Marzo, 2017.
by the body over time, and the adaptation of the material
response to the body is decisive. Biocompatibility then
assesses the biological activity of the material after being in
contact with human tissue. Accordingly, a biocompatible
material must not be genotoxic, cytotoxic, mutagenic,
carcinogenic, or immunogenic [3]. Biomaterials have been
developed in four different generations, and they are grouped
according to clinical requirements. Inert biomaterials have
been developed and selected since 1950 to enhance repair
processes or replacement of the host tissue without causing
an adverse reaction in the biological system; these materials
were called first-generation biomaterials. They included
cobalt alloys, alumina, and stable polyurethane and had great
importance in orthopedic and dental applications.
The second generation of biomaterials started due to poor
adhesion and implant loosening, which called for the
development and use of materials with bioactive surfaces
capable of forming complex bonds with the tissue. At the same
time, the use of biodegradable materials was also being
researched; thus, since 1970, materials with bioactive or
biodegradable properties have been studied. Titanium and
calcium phosphates, including hydroxyapatite, are examples of
second-generation biomaterials. However, the materials
mentioned above showed limited durability over time. Further
research is based on finding temporary materials capable of
being degraded in a controlled manner over time and promoting
a natural integration of the implant with the tissue that they
should eventually replace. Bioactivity and biodegradation
properties of third-generation materials such as magnesium and
its alloys are currently under study for tissue regeneration and
cardiovascular stents [1]. Since around 2010, researchers began
studying a new generation of biomaterials. This latest
generation of biomaterials encompasses biomimetic
nanocomposites for applications in tissue engineering [4]. Fig.
1. shows the four generations of biomaterials [5].
Currently, biomaterials science is focused on the
development of temporary degradable structures allowing
tissue integration with the implant, in order to improve the
replacement or repair process. After repair, the implant is
degraded and it is replaced by host tissue. Consequently,
degradable biomaterials for bone repair are generating much
interest —because they can save additional implant removal
procedures, reducing pain and health care costs.
On one hand, biodegradable or resorbable polymers such as
poly(lactic-co-glycolic acid) (PLGA), polylactic acid (PLA), and
polyglycolic acid (PGA) are currently being clinically applied.
However, they have shown adverse effects in the human body
when the immune system triggers osteolytic reactions around
Figure 1. Evolution of biomaterials.
Source: adapted from Bedilu et al. 2012.
the implant after a foreign body —for example, a
polymer— is recognized [6,7].
On the other hand, magnesium alloys are a new class of
promising materials currently under development [8].
Magnesium alloys exhibit poorer corrosion resistance in Cl-containing physiologic environments. Thus, these alloys
could be used as biodegradable metals. Moreover,
magnesium is essential to human metabolism, and it is the
fourth most abundant cation in the human body [9].
Biodegradable magnesium-based materials usually have a
high corrosion or biodegradation rate. Since this degradation
occurs in the early stages of implantation [10,11], this is a
limitation for using it in biomedical implants. The high
degradation rate of Mg presents several problems: 1.
Production of subcutaneous hydrogen bubbles during the
corrosion of the metal can cause accumulation of hydrogen
around the implant. Hydrogen can affect the healing process
and may lead to tissue necrosis due to formation of gaps
between the implant and the tissue [10,12,13]. 2. Changes in
local pH values caused by generation of OH- anions during the
cathodic reaction of the corrosion process [14]. This process
affects some cellular functions and cell viability [10]. 3. The
high corrosion rate of magnesium also produces an early
degradation of transverse sections of the implants. Due to the
fact that the time required by the bone to regenerate tissue is
between 3 and 4 months approximately. During that time the
implant must maintain mechanical integrity [15]. Nevertheless,
Mg implants will not withstand the mechanical stresses to
perform osseointegration if they are corroded. In the literature,
some authors have reported that the magnesium degradation
period is shorter than the period required to achieve mechanical
integrity [16]. Still, the time for the implant to be stable depends
on several factors, such as type of study (in vivo and in vitro
studies), material composition, implant location, type of model
(different animals), and days of evaluation of the in-vivo tests.
Moreover, since several methods are available for measuring
degradation rates, it is difficult to obtain correlations with in
vitro tests. As a result, nowadays there is great interest in the
surface properties of magnesium and its alloys to achieve
enhanced corrosion resistance and biocompatibility while
retaining the bulk properties of the material [17].
The corrosion rate of magnesium and magnesium alloys is a
key factor for the development of new biomaterials to be used in
resorbable implants. Therefore, coatings have emerged as a
possible solution, enabling the reduction of degradation at the
early stages of implanted materials and positively affecting the
tissue recovery process. Surface modification of magnesium can
be implemented to prevent electrolytes in the biological medium
or in electrolyte solutions from degrading the material quickly.
Titanium dioxide is a ceramic that has been widely used as a
coating material, mainly because it improves the corrosion
resistance of the surface-covering material (generally metals and
alloys) [18,21]. Although magnesium is a bio-inert material,
several authors have argued that it favors the formation of
calcium phosphates on its surface, which gives the material
bioactive properties that can be used in bone applications [22,23].
Furthermore, the photocatalytic activity of titanium dioxide
provides antibacterial properties [24].
This paper reviews magnesium modified with Titanium
dioxide, but it does not focus on the techniques since there
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Cathodic: Evolution of hydrogen
2
2
→
2
(2)
________________________________________________
Net reaction:
Figure 2. Degradation of magnesium in a biological environment.
Source: adapted from Wu, et al. 2013.
are several reviews already available in the literature
[11,13,25]. Its main goal is to gather information concerning
the development of titanium coatings to improve the
properties (corrosion rate and biocompatibility) of
magnesium for applications in biomaterials. In addition, the
importance of sol-gel techniques for biomedical coatings is
highlighted.
2. Magnesium
Magnesium is a metal of interest for bone applications
due to the fact that its density (1.74 g / cm3) and its Young’s
modulus of 45 GPa are very similar to those of human bone
(1.8-2.1 g / cm3 and 40-57 GPa respectively). Magnesium is
also very abundant in the human body, and it is essential for
cells, mainly because it takes part in various metabolic
processes inside the body. Additionally, it is known as an
essential ion since it participates in the formation of apatite
in the bone matrix. Magnesium implants were used for the
first time in 1878, but it was not until 1990 that they were
used again in orthopedic applications, when Erwin Payr [26]
introduced fixator pins, pegs, wires, and nails based on
biodegradable Mg for treatments on bone fractures. When
magnesium alloys are used for orthopedic implants, they
provide adequate mechanical properties, low stress shielding
effects (to avoid bone resorption), good biocompatibility, and
a controlled degradation rate [27]. However, interest in
implants made of magnesium and its alloys gradually
decreased for a period of time, because they are highly
susceptible to rapid corrosion in physiological environments,
limiting their application [28]. This accelerated corrosion
leads to premature failure of implants, since the support
needed for tissue healing is not provided. Furthermore,
production of large amounts of subcutaneous hydrogen
bubbles and modification of local pH could eventually lead
to tissue necrosis, as it was shown in a review by Frank Witte
[28]. Fig. 2. presents the natural process of degradation of
magnesium in a physiological environment [5].
Generally speaking, corrosion of magnesium occurs in
aqueous solutions and various redox reactions are present,
depending on the alloying elements. It is known that
hydrogen and magnesium hydroxide will appear during the
process. The most common corrosion reactions of Mg are
shown in eq. (1-3) [30].
Anodic: Dissolution of metal
→
2
(1)
2
→
(3)
Given that magnesium is (electrochemically) one of the
most active metals and that it consequently has high
corrosion rates, several alloys have been developed to
overcome those issues and promote its use in numerous
applications. Therefore, magnesium alloys are part of the
third generation of biomaterials and they can be classified in
different families: Mg-Al, Mg-Ca, Mg-Sr, Mg-Zn, Mg-Si,
Mg-Sn, Mg-Mn, Mg-Re, and Mg-Ag [31,32]. The most
commonly used commercial alloys are AZ (Mg-Al-Mn), AM
(Mg-Al-Mn), AE (Mg-Al-Re), the EZ series (Mg-Zn-Re),
ZK (mg-Zn-Zr), WE (Mg-RE-Zr), or AX AXJ (Mg-Al-Ca),
and the AJ series (Mg-Al-Sr) [33]. Different alloys have
particular advantages for certain applications and their
properties have been extensively evaluated. The literature
reports that the corrosion rate increases as temperature
increases and it decreases with aluminum content [34, 35].
Pardo et al. [36] assessed the corrosion behavior of
commercial alloys AZ31 (3.1% Al, 0.73% Zn, 0.25% Mn),
AZ80 (8.2% Al, 0.46% Zn, 0.13% Mn), and AZ91D (8.8%
Al, 0.68% Zn, 0.30% Mn). The authors concluded that an
increase in aluminum concentration significantly reduces the
corrosive activity of pure magnesium (99%). They also
determined that the AZ31 alloy is more corrosion-resistant in
a solution of sodium chloride (NaCl).
Since 2000, multiple original papers and reviews have
reported that magnesium has no adverse effects on the human
body —mainly because Mg is a trace element of the human
body— and the corrosion products can be considered
physiologically beneficial. Yun et al. [37] evaluated the
biocompatibility of Mg under corrosion effects in
osteoblasts. The mineralization and cytotoxicity results were
not significantly affected by the presence of corroded Mg. In
a comparative study by Witte et al. [28], four commercial
alloys — AZ31, AZ91, WE43, and LAE442— were
implanted in guinea pig femurs for 6 and 18 weeks,
demonstrating the osteoinductive capacity of Mg-based
materials. Recently, Zhen et al. [38] used the murine
fibroblasts cell line (L929) to test cell viability in different
concentrations of Mg2+ at various pH values in a DMEM
medium, and the results show that there is a considerable
hemolysis rate. However, the result was found for pH values
higher than 11, which is above the average physiological pH
(pH = 7). Additionally, the viability of cells treated with low
concentrations of Mg2+ (<100 µg / mL) is above 80%,
showing no significant toxicity to L929 cells.
Currently, the possible medical applications of
magnesium have gained more attention [31]. Therefore,
magnesium has been used for the manufacture of
biodegradable stents [39,40], in order to avoid diseases such
as thrombosis and restenosis via drug delivery [41]. Great
efforts to study biodegradable magnesium —in the form of
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implants for orthopedic applications— are focused on
improving the corrosion resistance by surface modification in
order to provide mechanical stability to the material. This is
done by favoring the initial attachment and subsequent
degradation that assist the cicatrization processes.
3. Coatings for magnesium implants
The surfaces of biomaterials implanted in the human body
are very important because they are in direct contact with host
tissue. Additionally, since reactions take place at the
interface, the capability of the material to be used as
biomaterial is determined by its surface.
Due to the fact that most materials are not biocompatible,
several methods have been used to modify their surface
properties, in order to improve biocompatibility,
biofunctionality, topography, and physical-chemical
properties, enabling the materials to maintain their bulk
material properties. That can be achieved with surface
modification, integration, and mechanical attachment to host
tissue [25]. Besides, the modified surface can be an effective
barrier against the physiological environment.
Coatings emerge as possible solutions to highly corrosive
media, forming films or layers on materials (ceramic or
polymeric materials). A study by Tang et al. [42] compared
in-vitro and in-vivo Mg-Zr coatings obtained by micro arc
oxidation (MAO) and electrophoretic deposition (EPD). The
authors established that EPD coatings are more corrosionresistant than coatings obtained by MAO. The results were
attributed to differences in the coating structure, mostly
because cracks or pores were not observed on the surface of
EPD coatings, while micropores were fully distributed on the
surface of MAO coatings. However, the cell-material
interactions were not considered in that study. Although the
results are satisfactory, the insufficient adhesion of coatings
is the main obstacle of the EPD technique and its potential
application. Subsequent studies [43] proposed a coating
method for the micro arc oxidation and liquid phase
deposition in order to obtain bioactive coatings on a Mg-ZnCa alloy. The authors stated that an evident improvement was
obtained in the substrate corrosion resistance, although the
coating obtained by MAO was porous.
Several polymers have been used as coatings because
they enhance the corrosion rate of magnesium and also have
acceptable biocompatibility. Witecka et al. [44] reported that
four different biodegradable polymers (PLGA, PLLA,
PHBV, and PHB) on Mg-2.0Zn-0.98Mn magnesium alloys
obtained by spin-coating successfully reduced the corrosion
of the substrate. Likewise, the results indicated that both
cytocompatibility and cell functionality were improved. Ma
et al. [45] have reviewed polymeric coatings for magnesium
and magnesium alloys and gathered additional information.
Ceramic coatings have also been successfully produced.
On one hand, Ren et al. [46] used a microwave-assisted
coating process to obtain a ceramic coating (HA and calcium
deficient hydroxyapatite -CDHA) on a AZ31 magnesium
alloy substrate. The authors fabricated uniform coatings in
less than 10 minutes, and the results showed that those
coatings improved corrosion behavior, they had good
cytocompatibility and promoted cellular proliferation after 5
days of incubation in a cell culture medium. On the other
hand, ceramic coatings were fabricated by micro arc
oxidation (MAO) on ZK61 magnesium alloys by Yu et al
[47]. The authors conclude that corrosion resistance was
improved by the coating, and that the latter favored the
precipitation of bone-like apatite on the coating surface after
immersed in SBF for 14 days.
Other authors have studied nanostructured coatings on
magnesium alloys for biomedical applications. Recently, a
novel bi-layered nanostructured SiO2/Ag-FHAp coating on
biodegradable magnesium has been successfully synthesized
by physical vapor deposition (PVD) combined with
electrodeposition (ED). The results showed high corrosion
resistance and biocompatibility [48].
3.1. Magnesium coated with titanium dioxide
Mechanical and biological fixation to the surrounding
bone is one of the major concerns regarding orthopedic
implants. Therefore, current research is focused on surface
modifications to improve the material’s osteoconductivity, to
have it act as a structural support and to promote new bone
formation and growth. Osteoconductivity must be
complemented with biodegradability because implant
materials must be reabsorbed or biodegraded into the body,
in order to enable the new tissue to grow and to provide an
adequate support [49]. Some calcium phosphates (CaP) —
including hydroxyapatite (HA)— have been proposed to
improve osseointegration of metal implants [50]. However,
titanium dioxide has emerged as a potential candidate to
improve the magnesium surface properties and corrosion
resistance.
Titanium dioxide (TiO2) and titanium oxide (IV) are
metal oxides. TiO2 crystallizes naturally in three main forms:
rutile, anatase, and brookite. The structure of this oxide is
based on a titanium atom surrounded by six oxygen atoms in
a distorted octahedral configuration [51]. Titanium oxide is
stable and non-toxic to the environment or humans. It also
has high catalytic activity, high oxidizing power, and it is
stable against photocorrosion [52]. Furthermore, it presents
antibacterial properties [24]. Different authors have reported
that titanium oxide induces the precipitation of bone-like
apatite or calcium phosphates on its surface [53-55]. Those
properties make titanium dioxide a suitable candidate for
bone replacement and reconstruction.
Coatings with titanium dioxide are well known to have
antibacterial properties. Guo et al. [56] observed the biocidal
effect of a porous coating with nanoparticles containing Ag
and TiO2, obtained by a sol-gel route. Fu and coworkers [57]
observed the biocidal effect as well when using titanium
dioxide coatings obtained by sol-gel methods on glass
substrates. In general, the antibacterial effect of TiO2 has
been extensively researched and the biological mechanism by
which this happens is currently under study [24,58]
Due to their mechanical properties and biocompatibility,
the uses of titanium dioxide coatings in biomedical sciences
include drug delivery systems [59-61] and dental and
orthopedic applications [62-65].
Since adequate interaction between the biomaterial and
the surrounding tissue, as well as biodegradability, are
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desired properties of an orthopedic implant, magnesium
alloys coated with titanium dioxide are of interest to
biomedical sciences. By combining those properties, the
lifetime of a magnesium implant can be extended and various
biological processes can be fostered without waiving the
biodegradation properties.
Titanium dioxide has been used as protective coating on
titanium substrates, 316L stainless steel, and other metals
[66], [67]. In spite of this, it is not easy to deposit TiO2 layers
onto magnesium alloys due to the low adhesion and low
homogeneity that some methods of surface modification
provide [68]. Nevertheless, several authors have successfully
managed to apply such coatings on magnesium and some of
those studies are mentioned in the following paragraph.
In their work, Marin et al. [69] used the atomic layer
deposition (ALD) technique for four different coatings. TiO2
/ aluminum mono and multilayers were applied to the
magnesium AZ31 substrate. The results showed that ALD
deposition can be successfully used to protect the AZ31 Mg
alloy from corrosion in aqueous solutions with low
concentrations of NaCl. It is important to highlight that
multilayer structures were the most effective against
corrosion but long deposition times are the main limitation to
this technique.
Ceramic coatings containing titanium dioxide
nanoparticles and organic compounds —such as alginate—
were placed on the magnesium AZ91D alloy by
electrophoretic deposition (EPD), thus improving the
corrosion resistance from 3 to 7 times when compared to
uncoated magnesium. The results were obtained by
electrochemical impedance spectroscopy (EIS) [70]. Hu et al.
[68] produced titanium dioxide coatings onto AZ31 alloys by
modifying the surface, using liquid phase deposition (LPD)
followed by an annealing process. The TiO2 coatings —with
or without annealing treatment— decreased the magnesium
alloy degradation rate, compared to uncoated magnesium in
electrochemical tests. Fujita and coworkers [71] researched
the corrosion resistance of TiO2 commercial films on Mg by
liquid phase deposition (LPD) and they found that by
changing the solution’s pH —from acidic to highly alkaline
along with the addition of sucrose— it becomes possible to
form a highly adhesive and thin TiO2 film on pure
magnesium without any heat treatment.
Recently Chen et al. [18] produced layers of
polydopamine (PDA) between the TiO2 coatings and the Mg
substrate to improve magnesium corrosion resistance.
Covalently-immobilized PDA layers on Mg and TiO2 were
obtained by deposition in liquid phase. The hybrid coating
exhibited a significantly lower corrosion current, as well as a
very low rate of in-vitro degradation (up to 21 days) in
phosphate-buffered saline (PBS), compared to plain TiO2
coatings.
Meanwhile, Li et al. [72] studied the mechanical
degradation of Mg-Ti composites in Hanks solution. For that
purpose, they used titanium wires embedded in the
magnesium matrix, employing the infiltration casting
technique. The authors concluded that magnesium in the
composite degrades more rapidly than pure magnesium.
However, the strength of the composite was maintained at
about 86 MPa after the Mg dissolved. Conversely,
Bakhsheshi-Rad et al. [73] synthesized a nanostructured
coating with hardystonite (HT) and titania/hardystonite onto
Mg alloys. The results indicate that the degradation resistance
of the magnesium alloys was improved, compared to an
uncoated magnesium sample. It is emphasized that the cell
viability was not affected by the exposure of the Mg coating
with titania/hardystonite to preosteoblastic cells (MCT3-E1).
Thus, the authors proposed new techniques to obtain coatings
on the substrate, in order to control the corrosion rate. Similar
data have been reported by Abdal-hay and coworkers [74]:
titanium dioxide / poly (lactic acid) coatings on magnesium
were used for biomedical applications, and they also showed
a significant reduction of the biodegradation rate and
hydrogen evolution of the magnesium substrate. These
coatings were fabricated by electron beam physical vapor
deposition (EB-PVD).
Micro arc oxidation has also been used to obtain porous
surfaces of magnesium, followed by a TiO2 coating for
sealing pores through the sol-gel method [75]. The authors
conclude that the resistance to biodegradation in Hanks
solution increases significantly —approximately 30 times—
compared to uncoated magnesium only. The parameters of
corrosion resistance were determined by electrochemical
impedance spectroscopy and polarization assay after 12
hours of immersion in the solution. The authors also
demonstrated that the coating provided a more stable surface
over time.
Similarly, the study of coatings with nano silica-titanium
dioxide on a Mg-Ca alloy by means of physical vapor
deposition (PVD) has been reported. Coatings based on nano
silica only were also evaluated. Electrochemical studies in
simulated body fluid (SBF) showed that the nano Si / TiO2
surface presents a significant reduction in corrosion rates
(0.57 mm / year), when compared to the sample coated with
nano silica only (0.91 mm / year), or the uncoated sample
(6.21 mm / year). Likewise, a low hydrogen evolution was
also observed [76].
3.2. Sol-gel
The sol-gel method is an efficient alternative to obtain
coatings on different materials, since it provides low-cost
coatings with good adhesion [77]. The method also enables
to control the structure of the materials at the nanoscale and
it can be used to produce metal oxides.
Such method starts with the formation of a colloidal
suspension of a molecular precursor in a solvent (sol) and a
subsequent oxide network formation produced at low
temperatures. The process involves two phases, signaled by
the hydrolysis and condensation of the suspension and a
polycondensation reaction that results in a three-dimensional
network. The solvent is then removed from the gel by a
drying process at room temperature (aging). Then, a heat
treatment is applied to obtain monoliths or thin films. The
sol-gel transition depends on the amount of precursors, water,
catalyst, temperature and pH [78].
Several authors have used sol-gel methods for
synthesizing TiO2 coatings. For example, Wang et al. [79]
proposed a new method of dip/spin coating by sol-gel to
prepare ultrafine titanium oxide films on α-Al2O3 disks. The
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method was different from others since the substrate was first
immersed in the sol and it was subsequently removed to form
a thin coating by spinning. The authors obtained small glass
films with uniform grains and a unique structure. Advincula
et al. [80] employed a sol-gel process to prepare titanium
oxide ultrafine silicon wafers. Those coatings were later
modified by self-assembled monolayers (SAM) of silanes
with different functional groups, in order to evaluate their invitro protein adsorption behavior. The sol-gel process
presents the advantage of accurately controlling the
nanostructure and the thickness of these thin coatings. The
authors suggested that surface modification with TiO2 by solgel and SAMs can promote protein adsorption in vitro, due
to changes in the surface chemical composition, roughness,
and wettability. Subsequently, Advincula et al. [81]
evaluated the biological response of the titanium oxide
matrix derived from sol-gel, reporting an increase in cell
adhesion and matrix mineralization on the substrates. The
adequate cellular response of titanium oxide by sol-gel has
been described by other authors [82,83]. In addition, the
literature has also reported [22,84] that sol-gel-prepared
titanium dioxide is able to induce calcium phosphates
formation on TiO2 surfaces, improving the adhesion of the
material to the biological tissue.
3.2.1. Surface modification of magnesium with titanium
dioxide by sol-gel
Titanium oxide coatings obtained by sol-gel have shown
promising results. Recently, Li et al. [85] evaluated the
corrosion resistance of coatings based on HA/TiO2 / PLA.
Titanium dioxide was used as an intermediate layer and it
provided high adhesion between the hydroxyapatite and the
poly (lactic acid). The surface modification was obtained by
the sol-gel technique through the dip-coating method, and the
corrosion resistance of the studied surface improved when
compared to uncoated substrates.
Different authors have evaluated surface modification of
magnesium with titanium dioxide by the sol-gel technique.
One of the studies was carried out by Ochsenbein et al. [83].
They analyzed the response of osteoblasts to different oxide
coatings obtained by sol-gel, and the results showed that
titanium dioxide fostered proliferation and cell adhesion after
three days of incubation. Another similar study performed by
Hu et al. [86] showed an anticorrosive effect of TiO2 coatings
obtained by sol-gel on an AZ31 magnesium alloy with
potential use in strong acidic environments. Furthermore, Hu
et al. [87] assessed the in-vitro degradation of magnesium
coated with a nano TiO2 film by sol-gel. They concluded that
the corrosion current decreased by almost 3 orders of
magnitude when compared to uncoated samples.
Li et al. [88] also showed that TiO2 coatings obtained by
the sol-gel method significantly improve the corrosion
resistance of a Mg-Ca alloy in SBF. More recent studies
evaluated titanium oxide and titanium oxide / calcium
phosphate coatings on AZ31 by the sol-gel method. Tang et
al. [89] observed that the coatings improved the corrosion
resistance and reduced the amount of hydrogen produced by
magnesium corrosion. Although hydrogen bubbles are
naturally created during degradation processes of magnesium
alloys, the amount of hydrogen produced can decrease when
the coating acts as a protection barrier. Corrosion studies
were carried out in the solution of simulated body fluid
(SBF). Moreover, Hernández et al [90] synthesized hybrid
coatings by a mixture of organic precursors, inorganic
glycidoxypropyltriethoxysilane
(GPTMS),
3aminopropyltriethoxysilane (APTES), and tetraethyl
orthosilicate (TEOS) by the sol-gel route, in order to increase
the corrosion resistance of the WE54-AE magnesium alloy.
Generally speaking, the results showed a significant
improvement in the corrosion current by about one order of
magnitude.
In biomedical engineering, the improvement obtained by
coatings on magnesium alloys can also be observed in the
increase of cellular activity. The literature reports few studies
on the biological properties of TiO2 substrates on
magnesium. Still, some reports are available. Amaravathy et
al. [91] successfully obtained sol-gel TiO2 coatings on a
magnesium alloy —AZ31— and the results showed a
significant improvement in adhesion during biological
assessments. In addition, coated Mg exhibited a higher cell
interaction (interaction area with the tissue) when compared
to the unmodified alloy. Similarly, Amaravathy el al. [92]
evaluated the coatings of titanium dioxide / hydroxyapatite
on the magnesium alloy AZ31. The results of hydrogen
evolution and potentiodynamic polarization studies showed
that the coatings can act as a protective layer, decelerating the
entry of corrosive metal ions from the SBF solution.
Furthermore, cell cultures revealed that biocompatibility and
cell adhesion of Mg coated with HA / TiO2 —evaluated by
the osteoblast cell line MG63— improved significantly,
which makes it a promising coating to induce
osteointegration properties. TiO2 coatings on magnesium
alloys usually create porous surfaces that may be unfavorable
for applications of magnesium as orthopedic biomaterial.
Surface defects may favor the rapid degradation of
magnesium alloys. This problem has been solved by
generating TiO2 multilayers on the Mg alloys. It has been
described that a chemically alternated multilayer
configuration generates greater resistance to corrosion [69].
Besides, it has also been reported that a correct densification
of the TiO2 film prevents the formation of surfaces with pores
or cracks [87]. New strategies include the use of bilayercoatings and hybrid multilayer coatings using TiO2 and
alumina [69] and TiO2 and hydroxyapatite [92] as coating
materials.
4. Conclusions
Surface modification techniques are an important area in
biomaterials research and biomedical engineering of
magnesium alloys. By surface modification of magnesium
and its alloys the biocompatibility can be enhanced and
problems related to degradation of magnesium can be
overcome while maintaining the bulk properties. Therefore,
this paper reviewed TiO2 coatings used to improve the
biological and corrosion resistance of magnesium and its
alloys.
Research carried out during the previous 5 years has
shown that the corrosion resistance and biological properties
266
Hernández-Montes et al / DYNA 84 (200), pp. 261-270, Marzo, 2017.
in the tissue-biomaterial interface of magnesium and its
alloys may be selectively enhanced using appropriate surface
treatments. Coatings obtained by sol-gel methods have
proven to be attractive due to their low processing costs and
because they enable to obtain homogeneous surfaces to
decrease the degradation rate of the implant. Some studies
have also reported that this technique can be used to create
antibacterial surfaces, if suitable precursors are used.
Although there are multiple surface modification
techniques to generate coatings on magnesium, considerable
research efforts are still needed to meet the current clinical
needs of biodegradable medical implants. Currently, there are
no coatings on magnesium capable of maintaining
mechanical integrity for the time needed by the bone tissue
to heal and subsequently be successfully degraded. More
efforts are needed to understand the interaction of
magnesium coated with TiO2 with biological systems (mainly
with cell lines derived from bone). There also are few studies
available in the literature with results of the performance of
in-vivo animal models. Likewise, studies showing
antibacterial properties of coatings on magnesium are still
under development.
Overall developments of new biodegradable magnesium
alloys for medical use are based on the control of the
corrosion rate and, thereby, the mechanical integrity of the
material in a physiological environment. Several research
papers published during the previous 5 years showed that
coatings with titanium dioxide significantly improve the
surface properties of magnesium. Surface treatments with
new and innovative materials can reduce the corrosion rate
and favor integration between the bone and the implant.
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V. Hernández-Montes, is a BSc. in Biomedical Eng. (2014), currently
completing a MSc. Eng. degree in Biomedical Engineering at Instituto
Tecnologico Metropolitano, Medellín, Colombia. She is a researcher for
Advanced Materials and Energy (MATyER) Research Group at Instituto
Tecnologico Metropolitano. Her interests include biomaterials with
emphasis on coatings and physical-chemical and mechanical
characterization of materials.
ORCID: 0000-0002-4692-3623
C. Betancur-Henao, is a BSc. in Biomedical Eng. (2014), currently
completing a MSc. Eng in Biomedical Engineering at Instituto Tecnologico
Metropolitano, Medellín, Colombia. She is a researcher for Advanced
Materials and Energy (MATyER) Research Group at Instituto Tecnologico
Metropolitano. Her interests include biomaterials with emphasis on
physical-chemical and mechanical characterization of materials.
ORCID: 0000-0003-1345-9091
J.F. Santa-Marín, is a BSc. in Mechanical Eng. (2005), MSc. Eng in
Materials and Process Engineering (2008) and PhD. in Engineering with
emphasis on science and technology of materials (2013) from Universidad
Nacional de Colombia. Currently, professor Santa is a researcher for
Advanced Materials and Energy (MATyER) Research Group at Instituto
Tecnologico Metropolitano. His interests include materials in general with
emphasis on wear, processing, and physical-chemical and mechanical
characterization of materials. He is coauthor of more than 12 international
publications in international journals such as Tribology International, Wear,
and others.
ORCID: 0000-0001-5781-672X
Área Curricular de Ingeniería
Geológica e Ingeniería de Minas y Metalurgia
Oferta de Posgrados
Especialización en Materiales y Procesos
Maestría en Ingeniería - Materiales y Procesos
Maestría en Ingeniería - Recursos Minerales
Doctorado en Ingeniería - Ciencia y Tecnología de
Materiales
Mayor información:
E-mail: [email protected]
Teléfono: (57-4) 425 53 68
270